A single-sided switch is a switching configuration where a switching device is used either to connect the load to a node at a lower potential—a “low-side” switch—or to a node at a higher potential—a “high-side” switch. The low-side configuration is shown in FIG. 1a, and the high-side configuration is shown in FIG. 2a, where the node at higher potential is represented by a high voltage (HV) source and the node at lower potential is represented by a ground terminal. In both cases, when the load 10 is an inductive load, a freewheeling diode 11 (sometimes referred to as a flyback diode) is required to provide a path for the freewheeling load current when the switching device is OFF. For example, as seen in FIG. 1b, when the switching device 12 is biased high by applying a gate-source voltage Vgs greater than the device threshold voltage Vth, current 13 flows through the load 10 and through switching device 12, and diode 11 is reverse biased such that no significant current passes through it. When switching device 12 is switched to low by applying a gate-source voltage Vgs<Vth, as shown in FIG. 1c, the current passing through the inductive load 10 cannot terminate abruptly, and so current 13 flows through the load 10 and through diode 11, while no significant current flows through switching device 12. Similar diagrams detailing current flow through the high-side switching configuration when the switch is biased high and when the switch is turned off (switched low) are shown in FIGS. 2b and 2c, respectively.
Ideally, the freewheeling diodes 11 used in the circuits of FIGS. 1 and 2 have low conduction loss in the ON state as well as good switching characteristics to minimize transient currents during switching, therefore Schottky diodes are commonly used. However, for some applications Schottky diodes cannot support large enough reverse-bias voltages, so high-voltage diodes which exhibit higher conduction and switching losses must be used. Switching devices 12, which are usually transistors, may be enhancement mode (normally off, Vth>0), also known as E-mode, or depletion mode (normally on, Vth<0), also known as D-mode, devices. In power circuits, enhancement mode devices are typically used to prevent accidental turn on, in order to avoid damage to the devices or other circuit components. A key issue with the circuits in FIGS. 1 and 2 is that most high voltage diodes typically exhibit high conduction and switching loss. Further, reverse recovery currents in high-voltage PIN diodes add to the losses of the transistor.
An alternative to the configurations illustrated in FIGS. 1 and 2 is to instead use synchronous rectification, as illustrated in FIGS. 3a-e. FIG. 3a is the same as FIG. 2a, except that a high-voltage metal-oxide-semiconductor (MOS) transistor 61 is included anti-parallel with diode 11. A standard MOS transistor inherently contains an anti-parallel parasitic diode and can therefore be represented as a transistor 62 anti-parallel to a diode 63, as illustrated in FIG. 3a. As seen in FIG. 3b, when switching device 12 is biased high and MOS transistor 61 is biased low, MOS transistor 61 and diode 11 both block a voltage equal to that across the load, so that the entire current 13 flows through the load 10 and through switching device 12. When switching device 12 is switched to low, as shown in FIG. 3c, diode 11 prevents transistor 62 and parasitic diode 63 from turning on by clamping the gate-drain voltage to a value less than Vth of the transistor and less than the turn-on voltage of the parasitic diode. Therefore, almost all of the freewheeling current flows through diode 11, while only a small, insignificant portion flows through the transistor channel and parasitic diode. As shown in FIG. 3d, MOS device 61 may then be biased high, which results in an increase in the channel conductivity of transistor 62 and thereby cause the majority of the freewheeling current to flow through the transistor channel. However, some dead time must be provided between turn-off of switching device 12 and turn-on of transistor 62 in order to avoid shoot-through currents from the high-voltage supply (HV) to ground. Therefore, diode 11 will be turned on for some time immediately after switching device 12 is switched from high to low and immediately before switching device 12 is switched back from low to high. While this reduces the conduction losses incurred by diode 11 in the absence of MOS transistor 61, the full switching loss for diode 11 is incurred, regardless of how long the diode remains on.
As shown in FIG. 3e, the circuit in FIGS. 3a-d can in principle operate without diode 11. In this case, parasitic diode 63 performs the same function that diode 11 performed in the circuit of FIGS. 3a-d. However, the parasitic diode 63 typically has much poorer switching characteristics and suffers from higher switching losses than a standard high-voltage diode, resulting in increased power loss, so the circuit of FIGS. 3a-d is usually preferred.
Many power switching circuits contain one or more high-side or low-side switches. One example is the boost-mode power-factor correction circuit shown in FIG. 4a, which contains a low-side switch. This circuit is used at the input end in AC-to-DC voltage conversion circuits. The configuration for the low-side switch in this circuit is slightly modified from that shown in FIG. 1a, since in FIG. 1a the freewheeling diode 11 is connected anti-parallel to the inductive load 10, whereas in this circuit the freewheeling diode 11 is between the inductive load 30 and the output capacitor 35. However, the fundamental operating principles of the two circuits are the same. As seen in FIG. 4b, when switching device 12 is biased high, current 13 passes through the load 30 and through the switching device 12. The voltage at the cathode end of the freewheeling diode 11 is kept sufficiently high by the output capacitor 35 so that the freewheeling diode 11 is reverse-biased, and thereby does not have any significant current passing through it. As seen in FIG. 4c, when switching device 12 is switched low, the inductor forces the voltage at the anode of the freewheeling diode 11 to be sufficiently high such that the freewheeling diode 11 is forward biased, and the current 13 then flows through the inductive load 30, the freewheeling diode 11, and the output capacitor 35. Because no significant current can flow in the reverse direction in a diode, diode 11 prevents discharge of the output capacitor 35 through switching device 12 during times where the load current is zero or negative, as can occur if the energy stored in the inductor 30 is completely transferred out before the commencement of the next switching cycle.